Thermally optimized phase change memory cells and methods of fabricating the same

ABSTRACT

A thermally optimized phase change memory cell includes a phase change material element disposed between first and second electrodes. The second electrode includes a thermally insulating region having a first thermal resistivity over the first electrode and a metallic contact region interposed between the phase change material element and the thermally insulating region, where the metallic contact layer has a second thermal resistivity lower than the first thermal resistivity.

CROSS REFERENCE TO RELATED APPLICATION

This patent application is a divisional of U.S. patent application Ser.No. 13/908,707 by Boniardi et al., entitled “Thermally Optimized PhaseChange Memory Cells and Methods of Fabricating the Same,” filed Jun. 3,2013, now U.S. Pat. No. 9,153,777, issued Oct. 6, 2015, assigned to theassignee hereof, filed on Jun. 3, 2013, now U.S. Pat. No. 9,153,777,issued Oct. 6, 2015, assigned to the assignee hereof, the disclosure ofwhich is incorporated by reference herein in its entirety.

FIELD

Subject matter disclosed herein relates to devices in integratedcircuits generally, and in particular, to devices incorporatingchalcogenide materials.

BACKGROUND

Devices incorporating phase change materials, e.g., chalcogenidematerials, such as for example switches and storage elements, may befound in a wide range of electronic devices. For example, devicesincorporating phase change materials may be used in computers, digitalcameras, cellular telephones, personal digital assistants, etc. Factorsthat a system designer may consider in determining whether and how toincorporate phase change materials for a particular application mayinclude, physical size, storage density, scalability, operating voltagesand currents, read/write speed, read/write throughput, transmissionrate, and/or power consumption, for example. Other example factors thatmay be of interest to a system designer include cost of manufacture,and/or ease of manufacture.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a three-dimensional illustration depicting a cross-pointmemory array according one embodiment.

FIG. 2A is an illustration depicting a cross-section of a phase changememory cell taken along a column line according to one embodiment.

FIG. 2B is an illustration depicting a cross-section of a phase changememory cell taken along a row line according to one embodiment.

FIG. 3A is an illustration depicting a cross-section of a phase changememory cell taken along a column line according to another embodiment.

FIG. 3B is an illustration depicting a cross-section of a phase changememory cell taken along a row line according to another embodiment.

FIG. 4A is an illustration depicting a cross-section of a phase changememory cell taken along a column line according to another embodiment.

FIG. 4B is an illustration depicting a cross-section of a phase changememory cell taken along a row line according to another embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

Devices incorporating phase change materials, e.g. memory devices, maybe found in a wide range of electronic devices. For example, devicesincorporating phase change materials may be used in computers, digitalcameras, cellular telephones, personal digital assistants, etc. Factorsrelated to devices incorporating phase change materials that a systemdesigner may consider in determining the device's suitability for aparticular application may include, physical size, storage density,scalability, operating voltages and currents, read/write speed,read/write throughput, transmission rate, and/or power consumption, forexample. Other example factors that may be of interest to a systemdesigner include cost of manufacture, and/or ease of manufacture.

In particular, memory devices incorporating phase change materials canprovide several performance advantages over other memory devices, suchas flash memory devices and dynamic random access memory devices (DRAM).For example, some phase change memory devices can be nonvolatile; i.e.,physical and electrical states of the memory devices change do notchange substantially over a retention time (e.g., longer than one year)without any external power supplied thereto. In addition, some phasechange memory devices can provide fast read and write access time (e.g.,faster than 10 nanoseconds) and/or high read and write access bandwidth(e.g., greater than 100 megabits per second). In addition, some phasechange memory device can be arranged in a very high density memoryarray, e.g., a cross-point array having greater than 1 million cells inthe smallest memory array unit connected with local metallization.

Performance of a phase change memory device with respect toabove-described characteristics depends on many factors. In particular,having good thermal isolation of the phase change material elementwithin a memory device and low electrical resistances between the phasechange material element and interfacing electrodes can reduce the energyrequired to program the device, as well as device-to-device thermaldisturbance (i.e., thermal cross-talk). In addition, having lowelectrical resistance between the phase change material and theinterfacing electrodes can also improve the signal-to-noise ratio of thememory device during a read operation. However, providing good thermalisolation can result in a tradeoff with low interfacial resistance, andvice versa. For example, interfacing electrodes often comprise metals,which can form low electrical resistance contacts with the phase changematerial element. However, such low contact materials tend to also begood thermal conductors, i.e., provide poor thermal isolation. Thus,there is a need for a thermally confined phase change memory devicehaving a phase change material element having low electrical resistanceand good thermal isolation. While embodiments are described herein withrespect to memory arrays, it will be understood that a thermallyconfined phase change memory device with reduced interfacial resistanceas described herein can also have application outside the memory arraycontext.

FIG. 1 shows a portion of a cross-point memory array 10 having N×M phasechange memory cells according to one embodiment of the presentinvention. The cross-point memory array 10 comprises first through Nthcolumn lines 20-1, 20-2, . . . , and 20-N, first through Mth row lines22-1, 22-2, . . . , and 22-M, and a plurality of memory cells disposedat at least a subset of the intersections formed by first through Nthcolumn lines and first through Mth row lines.

The cross-point memory array 10 includes access lines in the form offirst through Nth column lines 20-1, 20-2, . . . , and 20-N, which maybe referred to digit lines, e.g., bit lines (BLs). The cross-point array10 also includes crossing access lines in the form of first through Mthrow lines 22-1, 22-2, . . . , and 22-M, which may be referred to as wordlines (WLs). References to column lines and row lines, as well as theiralternative designations, are interchangeable. The coordinate axismarker 12 in this embodiment indicates that first through Nth columnlines 20-1, 20-2, . . . , and 20-N are extend along a y-direction (alsoreferred to herein as a column direction) and first through Mth rowlines 22-1, 22-2, . . . , and 22-M are oriented in a x-direction (alsoreferred to herein as a row direction). As illustrated, first throughNth column lines 20-1, 20-2, . . . , and 20-N are substantially parallelto each other. Similarly, the first through Mth row lines 22-1, 22-2, .. . , and 22-M are substantially parallel to each other. However, otherembodiments are possible, and word lines and digit lines can havenon-perpendicular orientations. Typically row lines are parallel to oneanother and column lines are parallel to one another at an angle suchthat they cross with the row lines. As used herein, the term“substantially” intends that the modified characteristic needs not beabsolute, but is close enough so as to achieve the advantages of thecharacteristic.

The cross-point memory array 10 further includes a plurality of memorycells disposed at at least a subset of the intersections formed by firstthrough Nth column lines and first through Mth row lines. In thisconfiguration, the cross-point memory array 10 includes up to N×M memorycells. In FIG. 1, however, for clarity, only four phase change memorycells are shown. As illustrated, the four phase change memory cells inthe example of FIG. 1 include first through fourth memory cells 30 a-dat intersections of nth column line 20-n and mth row line 22-m, nthcolumn line 20-n and (m+1)th row line 22-(m+1), (n+1)th column line20-(n+1) and mth row line 22-m, and (n+1)th column line 20-(n+1) and(m+1)th row line 22-(m+1), respectively. In this example, it will beunderstood that each one of the four memory cells 30 a-d has two nearestneighboring cells in the x-direction, two nearest neighboring cells inthe y-direction, and four next nearest neighboring cells in the diagonaldirection between the x-direction and the y-direction. For example, inthe x-direction, the first cell 30 a has two nearest neighboring cells,namely a fifth memory cell 30 e (not shown) at the intersection of(n−1)th column line 20-(n−1) and mth row line 22-m and the third memorycell 30 c at the intersection of (n+1)th column line 20-(n+1) and mthrow line 22-m. In addition, the y-direction, the first cell 30 a has twonearest neighboring cells, namely an eighth memory cell 30 h (not shown)at the intersection of nth column line 20-n and (m−1)th row line22-(m−1) and the second memory cell 30 b at the intersection of nthcolumn line 20-n and (m+1)th row line 22-(m+1). In addition, in the twodiagonal directions, the first cell 30 a has four next nearestneighboring cells, namely the fourth memory cell 30 d at theintersection of (n+1)th column line 20-(n+1) and (m+1)th row line22-(m+1), a sixth memory cell 30 f (not shown) at the intersection of(n−1)th column line 20-(n−1) and (m+1)th row line 22-(m+1), a seventhmemory cell 30 g (not shown) at the intersection of (n−1)th column line20-(n−1) and (m−1)th row line 22-(m−1), and a ninth memory cell 30 i(not shown) at the intersection of (n+1)th column line 20-(n+1) and(m−1)th row line 22-(m−1).

In one embodiment, column lines may comprise a suitable conductiveand/or semi conductive material including n-doped poly silicon, p-dopedpoly silicon, metals including Al, Cu, and W, conductive metal nitridesincluding TiN, TaN, and TaCN. In addition, top and bottom electrodes,described below, in various embodiments may comprise suitable conductivematerials including doped semiconductors, such as n-doped poly siliconand p-doped poly silicon, and/or metallic materials, such as metalsincluding C, Al, Cu, Ni, Cr, Co, Ru, Rh, Pd, Ag, Pt, Au, Ir, Ta, and W;conductive metal nitrides including TiN, TaN, WN, and TaCN; conductivemetal silicides including tantalum silicides, tungsten silicides, nickelsilicides, cobalt silicides, and titanium silicides; and conductivemetal oxides including RuO₂. In some embodiments, row lines may alsocomprise same or similar conductive and/or semiconductive materials ascolumn lines.

Each of the memory cells 30 a-d in FIG. 1 according to one embodiment isconfigured in a stack configuration to include first through fourthselector nodes 38 a-d on one of mth and (m+1)th row lines 22-m and22-(m+1), first through fourth bottom electrodes 36 a-d on selectornodes 38 a-d, first through fourth storage nodes 34 a-d on bottomelectrodes 36 a-d, and first through fourth top electrodes 32 a-d on thestorage nodes 34 a-d. Other embodiments of a stack configuration arepossible. For example, the positions of storage nodes 34 a-d andselector nodes 38 a-d within a stack configuration may be interchangedwith one another.

In one embodiment, each of the storage nodes 34 a-d includes a phasechange material. Suitable phase change materials include chalcogenidecompositions such as an alloy including at least two of the elementswithin the indium(In)-antimony(Sb)-tellurium(Te) (IST) alloy system,e.g., In₂Sb₂Te₅, In₁Sb₂Te₄, In₁Sb₄Te₇, etc., an alloy including at leasttwo of the elements within the germanium(Ge)-antimony(Sb)-tellurium(Te)(GST) alloy system, e.g., Ge₈Sb₅Te₈, Ge₂Sb₂Te₅, Ge₁Sb₂Te₄, Ge₁Sb₄Te₇,Ge₄Sb₄Te₇, etc., among other chalcogenide alloy systems. The hyphenatedchemical composition notation, as used herein, indicates the elementsincluded in a particular mixture or compound, and is intended torepresent all stoichiometries involving the indicated elements. Otherchalcogenide alloy systems that can be used in phase change storagenodes include Ge—Te, In—Se, Sb—Te, Ga—Sb, In—Sb, As—Te, Al—Te, In—Ge—Te,Ge—Sb—Te, Te—Ge—As, In—Sb—Te, Te—Sn—Se, Ge—Se—Ga, Bi—Se—Sb, Ga—Se—Te,Sn—Sb—Te, In—Sb—Ge, Te—Ge—Sb—S, Te—Ge—Sn—O, Te—Ge—Sn—Au, Pd—Te—Ge—Sn,In—Se—Ti—Co, Ge—Sb—Te—Pd, Ge—Sb—Te—Co, Sb—Te—Bi—Se, Ag—In—Sb—Te,Ge—Sb—Se—Te, Ge—Sn—Sb—Te, Ge—Te—Sn—Ni, Ge—Te—Sn—Pd, and Ge—Te—Sn—Pt, forexample.

When included in the memory cells 30 a-d, selector nodes 38 a-d may beelectrically coupled to storage nodes 34 a-d through bottom electrodes36 a-d on one side and electrically connected to row lines 22 on anotherside to form two terminal selector devices.

According to one embodiment, when selector nodes 38 a-d comprise achalcogenide material, the two terminal selector device can be an OvonicThreshold Switch (OTS). In this embodiment, a selector node may includea chalcogenide composition including any one of the chalcogenide alloysystems described above for a storage node. In addition, the selectornode may further comprise an element to suppress crystallization, suchas arsenic (As). When added, an element such as As suppressescrystallization by inhibiting any non-transitory nucleation and/orgrowth of the alloy. Accordingly, a selector node 38 a-d may beconfigured to switch to a conductive state when a potential exceeding athreshold voltage is applied across the selector node 38 a-d. Inaddition, the conductive state can be maintained while a sufficientholding current is maintained across the selector node. Examples includeTe—As—Ge—Si, Ge—Te—Pb, Ge—Se—Te, Al—As—Te, Se—As—Ge—Si, Se—As—Ge—C,Se—Te—Ge—Si, Ge—Sb—Te—Se, Ge—Bi—Te—Se, Ge—As—Sb—Se, Ge—As—Bi—Te, andGe—As—Bi—Se, among others.

According to another embodiment, selector nodes 38 a-d can form parts ofsemiconductor-based selector devices such as bipolar junctiontransistors (BJT). For example, in one embodiment, each one of selectornodes 38 a-d can be a p-type semiconductor to form a p-type emitterregion of a PNP BJT. Each one of selector nodes 38 a-d can be connectedto one of row lines 22, which can be an n-type semiconductor forming ann-type base region of the PNP BJTs. Each one of row lines 22 can in turnbe disposed on a p-type semiconductor, e.g., p-type substrate, forming ap-type collector region of the PNP BJT. In the embodiment of FIG. 1,selector nodes 38 a and 38 c forming p-type emitter regions areconnected by the row line 22-m forming a first common base region. Thefirst common base region can be contacted to an upper metal level at afirst base contact (not shown) farther along the x-direction. Similarly,selector nodes 38 b and 38 d forming p-type emitter regions areconnected by the row line 22-(m+1) forming a second common base region.The second common base can be contacted to an upper metal level at asecond base contact (not shown) farther along the x-direction. A personskilled in the art will understand that each of the row lines 22 canform a common base region connecting a suitable number of emitterregions. For example, the number of emitter regions in contact with acommon base region can be 4, 8, 16, 32, 64, or more. A person skilled inthe art will also understand that by analogy to a PNP BJT, an NPN BJTcan also be formed in a similar fashion. Additionally, in otherembodiments, p-type emitter regions are not connected to common baseregions.

In one embodiment, any one of the memory cells disposed at anintersection formed by any one of first through Nth column lines 20-1,20-2, . . . , and 20-N and first through Mth row lines 22-1, 22-2, . . ., 22-M may have a resistance state that may be a relatively highresistance state, also known as the RESET state, which can represent thestate of a phase change material in a storage node 34 that includes asubstantial amorphous region. Similarly, any one of the memory cells mayhave a resistance state that may be a relatively low resistance state,also known as the SET state, which can represent the state of a phasechange material in a storage node 34 that is substantially crystalline.Under this implementation, high and low resistance states may correspondto the “1” state and a “0” state in a single bit-per-cell memory system.However, the states “1” and “0” as they relate to high and lowresistance states may be used interchangeably to mean the opposite. Forexample, a high resistance state may be referred to as a “0” state, anda low resistance state may be referred to as a “1” state.

In other embodiments, any one of the memory cells disposed at anintersection formed by any one of the column lines and row lines mayhave a resistance state that may be an intermediate resistance state.For example, any one of the memory cells may have a resistance statethat is any one of first, second, third, and fourth resistance states,wherein the first resistance state is more resistive than the secondresistance state, the second resistive state is more resistive than thethird resistive state, and the third restive state is more resistivethan the fourth state. Under this implementation, first, second, third,and fourth resistance states may correspond to the “11,” “10,” “01”, and“00” states in a two bits-per-cell memory system. Yet other embodimentsare possible, where first through eighth resistance states represent thestates in a three-bits-per cell memory system, and where first throughsixteenth resistance states represent the states in a four-bits-per cellmemory system.

In one embodiment, each one of the memory cells disposed at anintersection formed by any one of first through Nth column lines 20-1,20-2, . . . , and 20-N and any one of first through Mth row lines 22-1,22-2, . . . , and 22-M may be accessed by an access operation. An accessoperation may be a write access operation, an erase access operation, ora read access operation. A write access operation, otherwise known asthe program operation or a RESET operation, changes the resistance stateof the memory cell from a relatively low resistance state to arelatively high resistance state. Similarly, an erase operation,otherwise known as the SET operation, changes the resistance state ofthe memory cell from a relatively high resistance state to a relativelylow resistance state. However, the terms “write” and “erase” as theyrelate to RESET and SET operations may be used interchangeably to meanthe opposite. For example, an erase operation may be referred to as aSET operation, and a program or write operation may be referred to as aRESET operation.

In an embodiment, each one of the memory cells disposed at anintersection formed by any of the column lines and row lines may beaccessed individually in a bit-addressable access mode. In abit-addressable access mode, a memory to be accessed may be a memorycell 30 a located at an intersection formed by a selected nth columnline 20-n and a selected mth row line 22-m. An access voltageV_(ACCESS), which may be a SET access voltage V_(SET), a RESET accessvoltage V_(RESET), or a read access voltage V_(READ), may be appliedacross the memory cell 30 a of this example by applying the accessvoltage across the selected nth column line 20-n and the selected mthrow line 22-m.

In one embodiment, a memory cell such as the memory cell 30 a at theintersection of the selected column line 20-n and the selected row line22-m accessed while preventing the remaining cells from gettingaccessed. This can be achieved by applying a voltage V_(ACCESS) acrossthe memory cell 30 a while allowing for voltages substantially lowerthan V_(ACCESS) to be applied across the rest of the cells, for examplememory cells 30 b-d. In one embodiment, this is obtained by applyingV_(ACCESS) to one end of the selected column line (nth column line 20-nin this example) while keeping one end of the selected row line (mth rowline 22-m in this example) at a low voltage V_(LOW), which may be atground potential. Concurrently, a voltage V_(COL INHIBIT) is appliedacross all remaining column lines (first through 20-(n−1) and 20-(n+1)through 20-N column lines in this example). In addition, a voltageV_(ROW INHIBIT) is applied across all remaining row lines (first through20-(m−1) and 20-(m+1) through 20-M row lines in this example). Underthis configuration, a voltage of about V_(ACCESS) is dropped between thenth column line 20-n and the mth row line 22-m across the memory cell 30a (which may be referred to hereinafter as a “target cell”). Inaddition, a voltage of about (V_(ACCESS)−V_(ROW INHIBIT)) is droppedacross inhibited cells such as the memory cell 30 b along the selectednth column line 20-n (which may be referred to hereinafter as “A-typecells”), and a voltage of about V_(COL INHIBIT) is dropped acrossinhibited cell such as the memory cell 30 c along the selected mth rowline 20-m (which may be referred to hereinafter as “B-type cells”). Inaddition, a voltage approximately equal to(V_(COL INHIBIT)−V_(ROW INHIBIT)) is dropped across all remainingdeselected cells such as the memory cell 30 d (which may be referred tohereinafter as “C-type cells”).

In one particular embodiment, V_(ROW INHIBIT) and V_(COL INHIBIT) isselected to be a voltage substantially equal to V_(ACCESS)/2. In thisimplementation, a voltage substantially equal to V_(ACCESS)/2 is droppedacross A-type cells (e.g. the memory cell 32 b) and across B-type cells(e.g. the memory cell 30 c) while a voltage substantially equal to zerois dropped across C-type cells (e.g. the memory cell 30 d). Thisembodiment may be utilized, for example, when the selector included inthe memory cell is an Ovonic Threshold Switch (OTS).

In another particular embodiment, V_(ROW INHIBIT) is selected to be atV_(ACCESS) and V_(COL INHIBIT) is selected to be at V_(LOW) (which maybe at ground potential). In this implementation, a voltage substantiallyequal to zero is dropped across type A cells (e.g. the memory cell 32 b)and across type B cells (e.g. the memory cell 30 c) while a voltagesubstantially equal to −V_(ACCESS) is dropped across C-type cells (e.g.the memory cell 30 d). This embodiment may be utilized, for example,when the selector included in the memory cell is a bipolar junctiontransistor (BJT).

Other embodiments are possible. For example, a voltage of V_(ACCESS)across the target cell can be obtained by applying a suitable positivefraction of V_(ACCESS) such as +½ V_(ACCESS) to one end of the selectedcolumn while applying a suitable negative fraction of V_(ACCESS) such as−½ V_(ACCESS) is applied to one end of the selected row. Similarly,suitable fractions of V_(ACCESS) can be chosen as V_(ROW INHIBIT) andV_(COL INHIBIT). A person skilled in the art will recognize thatchoosing a bias scheme depends on many factors, such as the selectordevice type, an overall cell current-voltage (IV) characteristics,number of columns, number of rows, and the overall array size, amongothers. A person skilled in the art will also recognize that the actualvoltages that similarly situated cells receive may deviate from thevoltage applied at one of the ends of a column or a row due to variousparasitic resistances and capacitances to which a particular cell may besubject to under a particular access condition.

FIGS. 2A and 2B illustrate cross-sectional views of the cross pointmemory array 10 of FIG. 1 viewed in a direction parallel to thex-direction and the y-direction, respectively. The cross sectional viewsare annotated with circuit representations demonstrating relevantresistive elements during an access operation of a target cell 30 a asdescribed above.

In FIG. 2A, for clarity, only two phase change memory cells are shownalong a column line 20 in the y-direction. As illustrated, the two phasechange memory cells include the first and second memory cells 30 a and30 b along a column line 20. As discussed in FIG. 1, each of the firstand second memory cells 30 a and 30 b is configured in a stackconfiguration to include first and second selector nodes 38 a and 38 bon a row lines 22, first and second bottom electrodes 36 a and 36 b onselector nodes 38 a and 38 b, first and second storage nodes 34 a and 34b on bottom electrodes 36 a and 36 b, and first and second topelectrodes 32 a and 32 b on storage nodes 34 a and 34 b.

Similarly, in FIG. 2B, for clarity, only two phase change memory cellsare shown along a row line 22 in the x-direction. As illustrated, thetwo phase change memory cells include the first and third memory cells30 a and 30 c along a row line 22. As discussed in FIG. 1, each of thememory cells 30 a and 30 c is configured in a stack configuration toinclude first and third selector nodes 38 a and 38 c on a row line 22,first and third bottom electrodes 36 a and 36 c on selector nodes 38 aand 38 c, first and third storage nodes 34 a and 34 c on bottomelectrodes 36 a and 36 c, and first and third top electrodes 32 a and 32c on storage nodes storage nodes 34 a and 34 c.

In addition, according to the illustrated embodiment in FIGS. 2A and 2B,adjacent memory cells in the y-direction can be interposed byinter-column dielectric regions 48 and adjacent memory cells in thex-direction can be interposed by inter-row dielectric regions 50. Theinter-column dielectric regions 48 and inter-row dielectric regions 40can be filled with suitable insulating material such as SiO₂ and Si₃N₄.Under these configurations, storage nodes are surrounded in x and ydirections by dielectrics and the array process architecture may bereferred to as fully-confined array architecture. When selector nodes 38a-d include emitter regions of BJTs, the inter-column dielectric regions48 and inter-row dielectric regions 50 adjacent to the selector nodes 38a-d can include shallow trench isolation dielectrics.

In the fully-confined array architecture of the cross-point memory array10 in FIGS. 2A and 2B, when accessed in a bit-addressable access mode,the memory cell 30 a to be accessed (i.e., the target memory cell) maybe memory cell 30 a located at an intersection formed by an nth column20-n and an mth row 22-m. An access voltage V_(ACCESS), which may be aSET access voltage V_(SET), a RESET access voltage V_(RESET), or a readaccess voltage V_(READ), may be applied across the target cell of thisexample by applying the access voltage V_(ACESS) across the a firstterminal 12 a and a second terminal 12 b of an access circuit path 12.First and second terminals 12 a and 12 b can represent ends of the nthcolumn 20-n and the mth row 22-m. V_(ACCESS) and the associated currentI_(ACCESS) will result in energy dissipation at various points along theaccess circuit path 12.

The access circuit path 12 includes first through third resistors 42,44, and 46 connected in electrical series between first and secondterminals 12 a and 12 b. When the storage node 34 a is in the SET state,the first through third resistors 42, 44, and 46 can have storage node34 a is in the RESET state, the first through third resistors 42, 44,and 46 can have first through third high resistance state (HRS)resistances R_(HRS1), R_(HRS2), and R_(HRS3), Contributions to LRSresistances R_(LRS1), R_(LRS2), and R_(LRS3) and HRS resistancesR_(HRS1), R_(LRS2), and R_(LRS3) can originate from various regionsacross the first memory cell 30 a. For example, regions that contributeto R_(LRS1) and R_(HRS1) can include a bulk material of the topelectrode 32 a and a first interface between the top electrode 32 a andthe storage node 34 a. Additionally, regions that contribute to R_(LRS2)and R_(HRS2) can include a bulk material of the storage node 34 a.Additionally, regions that contribute to R_(LRS3) and R_(HRS3) caninclude a second interface between the storage node 34 a and the bottomelectrode 36 a and a bulk material of the bottom electrode 36 a. It isto be understood that while other regions may also contributesubstantially to the overall resistance between the first and secondterminals 12 a and 12 b, only first through third resistors 42, 44, and46 are represented in the access circuit path 12 for clarity. Inaddition, other circuit paths can exist, for example, through the secondmemory cell 30 b, which are not shown nor discussed for clarity.

As a person skilled in the art will understand, in some implementations,an I_(ON)/I_(OFF) ratio can be an important consideration in designing amemory cell. An ON/OFF ratio can be proportional to the ratio(R_(HRS1)+R_(HRS2)+R_(HRS3))/(R_(LRS1)+R_(LRS2)−R_(LRS3)). WhenR_(HRS2)>>R_(HRS1)+R_(HRS3) and R_(LRS2)>>R_(LRS1)+R_(LRS3), the ON/OFFratio can be dominated by the ratio R_(HRS2)/R_(LRS2). Under thiscircumstance, the ON/OFF ratio can be desirably dominated by theresistance ratio of the bulk resistances of the storage node 34 a inRESET and SET states. On the other hand, whenR_(HRS2)<<R_(HRS1)+R_(HRS3) and R_(HRS2)<<R_(LRS1)+R_(LRS3), the ON/OFFratio can be relatively independent of the ratio R_(HRS2)/R_(LRS2).Under this circumstance, the ON/OFF ratio can be undesirably dominatedby the resistance values of the first and second interfaces between thestorage node 34 and the top and bottom electrodes 32 a and 36 a and/orthe resistance values of the bulk materials of the top and bottomelectrodes 32 a and 36 a. Thus, from an ON/OFF ratio point of view,relatively low LRS and HRS resistances R_(LRS1), R_(LRS3), R_(HRS1), andR_(HRS3) of the first and third resistors 42 and 46 and relatively highLRS and HRS resistances R_(HRS2) and R_(HRS2) of the second resistor 44can be preferred.

The energy efficiency of SET and RESET operations can also beproportional to heats generated at various regions of the memory cell.In some implementations, it may be desirable to have self-heating of thestorage node dominate over heats generated in other regions. From thisstandpoint, it may be desirable to have R_(HRS2)>>R_(HRS1)+R_(HRS3) andR_(LRS2)>>R_(LRS1)+R_(LRS3). When a voltage V_(ACCESS) is appliedbetween the first and second terminals 12 a and 12 b of the accesscircuit path 12, voltages proportional to respective LRS and HRSresistances can drop across first, second, and third resistors 42, 44,and 46. For example, when V_(ACCESS)=V_(RESET) is applied between thefirst and second terminals 12 a and 12 b, first, second, and thirdvoltages V_(RESET1), V_(RESET2), and V_(RESET3) can drop across first,second, and third resistors 42, 44, and 46, respectively. The resultingcurrent I_(RESET)=V_(RESET)/(R_(LRS1)+R_(LRS2)+R_(LRS3)) can result ingeneration of first, second, and third RESET heats Q_(RST1)=I_(RESET)²R_(LRS1) and Q_(RST2)=I_(RESET) ²R_(LRS2), and Q_(RST3)=I_(RESET)²R_(LRS3), respectively. Thus, for maximum energy efficiency of theRESET operation, it may be desirable to have R_(LRS2)>>R_(LRS1)+R_(LRS3)such that more of the access energy is spent on self-heating of thestorage node 34 a compared to heating the interfaces.

Similarly, when V_(ACCESS)=V_(SET) is applied between the first andsecond terminals 12 a and 12 b, first, second, and third voltagesV_(SET1), V_(SET2), an V_(SET3) can drop across first, second, and thirdresistors 42, 44, and 46, respectively. In addition, the resultingcurrent I_(SET)=V_(SET)/(R_(HRS1)+R_(HRS2)+R_(HRS3)) can result ingeneration of first, second, and third SET heats Q_(SET1)=I_(SET)²R_(HRS1) and Q_(SET2)=I_(SET) ²R_(HRS2), and Q_(SET3)=I_(SET)²R_(HRS3), respectively. Thus, for maximum energy efficiency of the SEToperation, it may also be desirable to have R_(HRS2)>>R_(HRS1)+R_(HRS3)such that more of the access energy is spent on self-heating of thestorage node 34 a compared to the interfaces.

The energy efficiencies of SET and RESET operations can also beproportional to confinement of heat in the storage node during SET andRESET operations. Accordingly, the performance of a memory cell can beimproved minimizing heat loss from the storage node. As illustrated inFIGS. 2A and 2B, there can be heat loss in six directions. A first heatQ₁ can be lost from the storage node 34 a in the z-direction towards thetop electrode 32 a. A second heat Q₂ can be lost from the storage node34 a in the z-direction towards the bottom electrode 36 a. A third heatQ₃ can be lost from the storage node 34 a in opposite y-directionstowards adjacent storage nodes 34 b and 34 h (not shown). A fourth heatQ₄ can be lost from the storage node 34 a in opposite x-directionstowards adjacent storage nodes 34 c and 34 e (not shown).

First through fourth lost heats Q₁-Q₄ can depend on many factors.Generally, one-dimensional heat flux can be expressed as beingproportional to −κ(dT/dx), where dT/dx is the temperature gradient onedimension in the direction of the heat flow and κ is the thermalconductivity of the heat transfer medium, which is inverselyproportional to the thermal resistivity of the heat transfer medium. Inthis connection, first through fourth heats Q₁-Q₄ can be characterizedas being inversely proportional to first through fourth thermalresistances R_(TH1)-R_(TH4) associated with first through fourth heatsQ₁-Q₄. In the fully-confined array architecture of the cross-pointmemory array 10, contributions to first through fourth thermalresistances R_(TH1)-R_(TH4) can originate from various regions connectedto the first memory cell 30 a. For example, regions that contribute toR_(TH1) can include a bulk material of the top electrode 32 a and afirst interface between the top electrode 32 a and the storage node 34a. Additionally, regions that contribute to R_(TH2) can include thesecond interface between the storage node 34 a and the bottom electrode36 a and a bulk material of the bottom electrode 36 a. Additionally,regions that contribute to R_(TH3) can include a third interface betweenthe storage node 34 a and adjacent inter-column dielectric region 48 aswell as a bulk material of the inter-column dielectric region 48.Additionally, regions that contribute to R_(TH4) can include a fourthinterface between the storage node 34 a and adjacent inter-columndielectric region 50 as well as a bulk material of the inter-columndielectric region 50. It is to be understood that while other regionsmay also contribute substantially to the overall thermal resistancesurrounding storage node 34 a, only first through fourth thermalresistances R_(TH1)-R_(TH4) are discussed for clarity.

Excessive loss of heat in the x and y directions can also lead tothermal disturb of adjacent cells during programming a target cell(sometimes referred to as program disturb). A program disturb occurswhen the heat generated by performing a SET operation or a RESEToperation on the target memory cell results in a heat transfer to anadjacent memory cell (sometimes referred to as a victim cell) such thatan adjacent cell in a RESET state at least partially transforms to a SETstate. As a general rule, the time for phase

$\begin{matrix}{t_{cryst} \propto {\exp\left( \frac{E_{A}}{k_{B}T} \right)}} & (1)\end{matrix}$transformation t_(cryst) of a storage node at a given temperature T canbe governed by an Arrhenius relationship

where E_(a) is the activation energy, k_(B) is the Boltzmann constant,and T is the temperature of the victim cell. Due to the exponentialnature of the crystallization kinetics, a small increase in thetemperature of the victim cell can lead to a substantial degradation ofprogram disturb time-to-fail. In addition, a time required to disturb avictim cell can be cumulative; i.e., while a single RESET operationhaving a RESET pulse duration of t_(RESET) may not be sufficient tocause a significant program disturb, repetition of many RESET operationscan result in a program disturb. In this connection, minimizing loss ofthird and fourth heats Q₃ and Q₄ in FIGS. 2A and 2B, which can beachieved by maximizing R_(TH3) and R_(TH4) discussed above, can lead toan improvement in program disturb performance of the cross-point memoryarray 10.

In the embodiment of FIG. 2A, the column line 20, top electrodes 32 aand 32 b, storage nodes 34 a and 34 b, bottom electrodes 36 a and 36 b,selector nodes 38 a and 38 b, and row lines 22 have first through sixthlateral dimensions in the y-direction d_(1a), d_(2a), d_(3a), d_(4a),d_(5a), and d_(6a), respectively. In addition, a seventh lateraldimension in the y-direction d_(7a) represents a spacing betweenadjacent memory cells 30 a and 30 b. The first lateral dimension in they-direction d_(1a) represents a column length of column lines 20 of thecross-point memory array 10. In one embodiment, a substantially similarsecond through sixth lateral dimensions in the y-direction d_(2a),d_(3a), d_(4a), d_(5a), and d_(6a), can result from patterning andetching within a single photo mask level a stack between and includingthe top electrodes 32 a and 32 b and row lines 22.

Similarly, in the embodiment of FIG. 2B, the column lines 20, topelectrodes 32 a and 32 c, storage nodes 34 a and 34 c, bottom electrodes36 a and 36 c, selector nodes node 38 a and 38 c, and row line 22 havefirst through sixth lateral dimensions in the x-direction d_(1b),d_(2b), d_(3b), d_(4b), d_(5b), and d_(6b), respectively. In addition, aseventh lateral dimension in the x-direction d_(7b) represents thespacing between adjacent memory cells 30 a and 30 c in the x-direction.The sixth lateral dimension in the x-direction d_(7b) represents a rowlength of row lines 22 in the cross-point memory array 10. In oneembodiment, a substantially similar first through fifth lateraldimensions in the x-direction d_(1b), d_(2b), d_(3b), d_(4b), and d_(5b)result from patterning and etching within a single photo mask level astack between and including the columns 20 and storage nodes 32 a and 32b.

The first lateral dimension in the y-direction d_(1a) representing acolumn length is a function of the number of row lines M the column line20 traverses in the y-direction. For example, in an array with M rowlines where d_(7a) represents the spacing between adjacent memory cellsin the y-direction, d_(1a) may be at least (M×d_(6a))+(M×d_(7a)).Similarly, the sixth lateral dimension in the x-direction d_(6b)representing a row length is a function of the number of column lines Nthe row line 22 traverses in the x-direction. For example, in an arraywith N column lines where d_(7b) represents the spacing between adjacentmemory cells in the x-direction, d_(6b) may be at least(N×d_(1b))+(N×d_(7b)).

In the fully-confined array architecture of the cross-point memory array10 according to one embodiment, d_(2a)−d_(6a) in FIG. 2A can be selectedto be the range between about 40 nm and 60 nm, for example 50 nm. Inanother embodiment, d_(2a)−d_(6a) can have dimensions selected to be therange between about 25 nm and 40 nm, for example 35 nm. In anotherembodiment, d_(2a)−d_(6a) can have dimensions selected to be the rangebetween about 18 nm and 25 nm, for example 20 nm. In yet anotherembodiment, d_(2a)−d_(6a) can have dimensions selected to be the rangebetween about 5 nm and 18 nm, for example 14 nm. Smaller dimensions areyet possible, limited only by the lithographic capability employed bythe person skilled in the art. Similar ranges of dimensions can bechosen for d_(1b)−d_(5b) in FIG. 2B.

According to the embodiments of FIGS. 2A and 2B, column lines 20 have afirst thickness h₁, the top electrodes 32 a-c have a second thicknessh₂, the storage node 34 a-c have a third thickness h₃, the bottomelectrode 36 a-c have a fourth thickness h₄, the selector nodes 38 a-chave a fifth thickness h₅, and row lines 22 have a sixth thickness h₆.In one embodiment, the first thickness h₁ has a thickness selected to bethe range between about 10 nm and 100 nm, for example 35 nm, the secondthickness h₂ has a thickness selected to be the range between about 10nm and 50 nm, for example 25 nm, the third thickness h₃ has a thicknessselected to be the range between about 5 nm and 50 nm, for example 25nm, and the fourth thickness h₄ has a thickness selected to be the rangebetween about 10 nm and 100 nm, for example 25 nm. In embodiments whereselector nodes 38 a-c comprise chalcogenide materials of OTS switches,the fifth thickness h₅ has a thickness selected to be the range betweenabout 5 nm and 50 nm, for example 25 nm, and the sixth thickness h₆ hasa thickness selected to be the range between about 10 nm and 100 nm, forexample 50 nm.

In connection with the discussion above, a person having ordinary skillin the art will understand that choosing materials coupled to storagenodes and selector nodes to optimize the performance of a memory cellwith respect to their electrical resistances and thermal resistances canhave tradeoffs. For example, in FIGS. 2A and 2B, choosing a metallicmaterial to serve as top and bottom electrodes 32 a and 36 a of thestorage node 34 a can desirably reduce first and third LRS and HRSresistances R_(LRS1), R_(LRS3), R_(HRS1), and R_(HRS3) However, such achoice can undesirably reduce first and second thermal resistancesR_(TH1) and R_(TH2). Embodiments that minimize such tradeoffs are nowdisclosed herein.

FIGS. 3A and 3B illustrate cross-sectional views of a portion of across-point memory array 80 viewed from a direction parallel to thex-direction and the y-direction, respectively. The cross-point memoryarray 80 is configured as fully-confined array architecture describedabove. The cross sectional views are annotated with a circuitrepresentation demonstrating relevant resistive elements during anaccess operation of a memory cell 50 a as described above. Similar tothe cross-point array 10 of FIGS. 2A and 2B, the cross-point array 80includes memory cells disposed between column lines 20 and row lines 22.

According to the illustrated embodiment in FIGS. 3A and 3B, a memorycell includes a bottom electrode, a chalcogenide material elementdisposed on the bottom electrode, and a top electrode disposed on thechalcogenide material element. The top electrode includes a topthermally insulating region over the first electrode, where thethermally insulating region comprises carbon and has a first thermalresistivity. Top electrode additionally includes a top metallic contactregion interposed between the chalcogenide material element and thethermally insulating region, where the metallic contact material has asecond thermal resistivity lower than the first thermal resistivity.

Additionally, the chalcogenide material element has top and bottomsurfaces and first and second sidewalls extending between top and bottomsurfaces, and the memory cell further includes a plurality of sidewallthermal insulators formed over first and second sidewalls.

In particular, the cross-point memory array 80 of FIGS. 3A and 3Bincludes memory cells 50 a-50 c. Memory cells 50 a-50 c include bottomelectrodes including bottom thermally insulating regions 60 a-60 c andbottom metallic contact regions 58 a-58 c disposed on row lines (notshown). In one embodiment, bottom thermally insulating regions 60 a-60 care disposed on row lines 22 and may include carbon. Bottom thermallyinsulating regions 60 a-60 c can include carbon in various forms,including graphitic carbon, diamond-like carbon, and amorphous carbon,among others. Bottom thermally insulating regions 60 a-60 c includingcarbon can be formed using various processing techniques, includingchemical vapor deposition, plasma-enhanced chemical vapor deposition,physical vapor deposition, among others.

In another embodiment, bottom metallic contact regions 58 a-58 c aredisposed on thermally insulating regions 60 a-60 c and comprise metallicmaterials. Bottom metallic contact regions 58 a-58 c include suitableconductive and/or semiconductive materials including n-doped polysilicon and p-doped poly silicon, metals including Al, Cu, Ni, Cr, Co,Ru, Rh, Pd, Ag, Pt, Au, Ir, Ta, and W, conductive metal nitridesincluding TiN, TaN, WN, and TaCN, conductive metal silicides includingtantalum silicides, tungsten silicides, nickel silicides, cobaltsilicides, and titanium silicides, and conductive metal oxides includingRuO₂.

In some embodiments, bottom thermally insulating regions 60 a-60 c arein direct contact with bottom metallic contact regions 58 a-58 c. Inother embodiments, intervening regions may be present that can includematerials in bottom thermally insulating regions 60 a-60 c and bottommetallic contact regions 58 a-58 c.

Additionally, memory cells 50 a-50 c include chalcogenide materialelements 56 a-56 c disposed on bottom metallic contact regions 58 a-58c. In one embodiment, chalcogenide material elements 56 a-56 c can bestorage nodes described in connection with FIGS. 2A and 2B. In thisembodiment, there can be selector nodes coupled elsewhere (not shown inFIGS. 3A and 3B), for example, coupled to bottom thermally insulatingregions 60 a-60 c. Selector nodes can be part of various selectordevices described above, including, for example, BJTs and OTSs. Inanother embodiment, chalcogenide material elements 56 a-56 c can beselector nodes described in connection with FIGS. 1, 2A, and 2B. In thisembodiment, there can be storage nodes coupled elsewhere (not shown inFIGS. 3A and 3B), for example, coupled to bottom thermally insulatingregions 60 a-60 c.

Additionally, memory cells 50 a-50 c include top electrodes includingtop thermally insulating regions 52 a-52 c and top metallic contactregions 54 a-54 c disposed on chalcogenide material elements 56 a-56 c.In one embodiment, top metallic contact regions 54 a-54 c are disposedon chalcogenide material elements 56 a-56 c and comprise metallicmaterials. Top metallic contact regions 54 a-54 c include suitableconductive and/or semiconductive materials similar to bottom metalliccontact regions 58 a-58 c described above.

In another embodiment, top thermally insulating regions 52 a-52 c aredisposed on top metallic contact regions 54 a-54 c and include carbon.Top thermally insulating regions 52 a-52 c can include carbon in similarforms and be formed using similar processing techniques as in bottomthermally insulating regions 60 a-60 c.

In some embodiments, bottom thermally insulating regions 60 a-60 c arein direct contact with bottom metallic contact regions 58 a-58 c. Inother embodiments, intervening regions may be present that can includematerials in bottom thermally insulating regions 60 a-60 c and bottommetallic contact regions 58 a-58 c.

In addition, in some embodiments, chalcogenide material elements 56 a-56c can be in direct contact with one or both of top and bottom metalliccontact regions 54 a-54 c and 58 a-58 c. In other embodiments,intervening regions may be present that can include materials inchalcogenide material elements 56 a-56 c and top and bottom metalliccontact regions 54 a-54 c and 58 a-58 c.

According to the illustrated embodiment of FIG. 3A, chalcogenidematerial elements 56 a and 56 b have top and bottom surfaces and firstand second sidewalls extending between top and bottom surfaces along thez-direction. In the fully-confined array architecture of the cross-pointmemory array 80, first and second sidewalls are separated in they-direction by a distance spanning one cell dimension in the y direction(which is similar to the third lateral dimension in the x-directiond_(3a) in FIG. 2A, not illustrated here for clarity). In thisconfiguration, memory cells 50 a and 50 b include first sidewall thermalinsulators 72 formed over first and second sidewalls. The first sidewallthermal insulators 72 includes a plurality of sidewall materials, e.g.,sidewall layers 72-1 to 72-n (not labeled individually in FIG. 3A forclarity), to provide sufficient thermal insulation to improveprogramming efficiency of the chalcogenide material element of a targetmemory cell, and to provide improved immunity against program disturb,as discussed above in connection with FIGS. 2A and 2B. In the exampleembodiment of FIG. 3A, the chalcogenide material element of the targetcell can be 56 a, and the chalcogenide material element of the victimcell can be 56 b adjacent to each other along a common column line 20.Each of the first sidewall thermal insulators 72 includes first throughnth sidewall layers. In the illustrated example, by way of an exampleonly, each of the first sidewall thermal insulators 72 includes firstthrough fifth sidewall layers 72-1-72-5 on the first and secondsidewalls, where the first sidewall layer 72-1 can be in contact withfirst and second sidewalls. According to one embodiment, first andsecond sidewall layers comprise first and second dielectric materials D1and D2, where each of the first and second sidewall layers have anatomic element not included in the other sidewall layer. According toanother embodiment, the sidewall layers can have an alternatingarrangement, such that each of first, third, and fifth sidewall layerscomprise D1 and each of second and fourth sidewall layers comprise D2(i.e., D1/D2/D1/D2/D1). Other embodiments are possible, where each thirdthrough fifth sidewall layers 72 c-72 e can comprise third through fifthdielectric materials D3-D5 that can be one of D1 or D2 or othermaterials.

In one embodiment, each of the first through nth sidewall layers 72-1 to72-n of first sidewall thermal insulators 72 can have a thicknessranging from about 1 to 10 nm, for instance about 2 nm. In anotherembodiment, each of the first through nth sidewall layers 72-1 to 72-nof the first sidewall thermal insulators 72 can have a thickness rangingfrom about 2 to 5 nm, for instance about 3.5 nm. In addition, in someembodiments, the thicknesses of sidewall layers 72-1 to 72-n aresubstantially the same. In other embodiments, the thicknesses ofsidewall layers 72-1 to 72-n are substantially different from oneanother.

In one embodiment, first sidewall thermal insulators 72 can have 1 to 20sidewall layers (i.e., n can be 1 to 20), for instance 10 sidewalllayers. In another embodiment, first sidewall thermal insulators 72 canhave 2 to 10 sidewall layers (i.e., n can be 2 to 10), for instance 6sidewall layers. In yet another embodiment, first sidewall thermalinsulators 72 can have 3 to 7 sidewall layers (i.e., n can be 3 to 7),for instance 5 sidewall layers.

In one embodiment, each of the first through nth sidewall layers of thefirst sidewall thermal insulators 72 can include oxides such as SiO₂,ZrO₂, HfO₂, Al₂O₃, NiO, TiO₂, Ta₂O₅, ThO₂, HfSiO₄, ZrSiO₄, Mg₂SiO₄, MgO,BeO, and oxides of lanthanide series, among other oxides. In anotherembodiment, each of the first through nth sidewall layers of the firstsidewall thermal insulators 72 can include nitrides or carbides such asSi₃N₄ and SiC, among other nitrides and carbides.

Each of the first through nth sidewall layers of first sidewall thermalinsulators 72 can be formed using a suitable deposition technique. Forexample, the first through nth sidewall layers can be formed usingvarious processing techniques, including atomic layer deposition,chemical vapor deposition, plasma-enhanced chemical vapor deposition,physical vapor deposition, among others. A person skilled in the artwill understand that the choice of a processing technique depends onmany factors, e.g., as the availability of the precursor material,feature size and/or aspect ratio of an opening through which thematerial to be deposited is to travel, cost, and conformality of thedeposited material, among others.

FIG. 3B similarly illustrates chalcogenide material elements 56 a and 56c having top and bottom surfaces and third and fourth sidewallsextending between top and bottom surfaces along the z-direction. In thefully-confined array architecture of the cross-point memory array 80,third and fourth sidewalls are separated in the x-direction by adistance spanning one cell dimension in the x direction (which issimilar to the third lateral dimension in the x-direction d_(3b) in FIG.2B, not illustrated here for clarity). In addition, third and fourthsidewalls are substantially perpendicular to first and second sidewalls.In this configuration, memory cells 50 a and 50 c include secondsidewall thermal insulators 74 formed over third and fourth sidewalls.Similar to first sidewall thermal insulators 72, second sidewall thermalinsulators 74 include a suitable number of sidewall materials, e.g.,layers, to provide sufficient thermal insulation to improve programmingefficiency of the chalcogenide material element of a target memory cell,and to provide improved immunity against program disturb, as discussedabove in connection with FIGS. 2A and 2B. In the example embodiment ofFIG. 3B, the chalcogenide material element of the target cell can be 56a, and the chalcogenide material element of the adjacent victim cell canbe 56 c along a common row line 22. Each of the second sidewall thermalinsulators 74 includes (n+1)th through mth sidewall layers (not labeledindividually in FIG. 3B for clarity). In the illustrated example, by wayof an example only, each of the second sidewall thermal insulators 74includes sixth through tenth sidewall layers 74-6 to 74-10 on third andfourth sidewalls, where the sixth sidewall layer 74-6 can be in contactwith third and fourth sidewalls. The materials, thicknesses, and thenumber of (n+1)th through mth sidewall layers of the second sidewallthermal insulators 74 can be similar to those described for firstsidewall thermal insulators 72. In addition, the sidewall layers of thesecond sidewall thermal insulators 74 can be formed using similartechniques as those described for first sidewall thermal insulators 72.

A person skilled in the art will understand that a choice of certainmaterial combinations for different regions of the memory cell and thearray can be made to optimize certain aspects of the performance of thememory cell. These aspects are discussed by referring to a circuit path14. In the fully-confined array architecture of the cross-point memoryarray 80 in FIGS. 3A and 3B, the memory cell 50 a may be accessed in abit-addressable mode by application of an access voltage V_(ACCESS)across the a first terminal 14 a and a second terminal 14 b of theaccess circuit path 14. The first and second terminals can represent annth column 20 and a mth row 22. V_(ACCESS) and the associated currentI_(ACCESS) will result in energy dissipation at various points along theaccess circuit path 14.

The access circuit path 14 includes first through third resistors 62,64, and 66 connected in electrical series between first and secondterminals 14 a and 14 b. Similar to the access circuit path 12 in FIGS.2A and 2B, the first through third resistors 62, 64, and 66 can havefirst through third LRS resistances R′_(LRS1), R′_(LRS2), and R′_(LRS3)and HRS resistances R′_(HRS1), R′_(HRS2), and R′_(HRS3). Contributionsto LRS and HRS resistances can originate from various regions across thememory cell 50 a, which is a target memory cell in this example. Forexample, regions that contribute to R′_(LRS1) and R′_(HRS1) can includea bulk material of the top thermally insulating region 52 a, a bulkmaterial of the top metallic contact region 54 a, and a first interfacebetween the top metallic contact region 54 a and the storage node 56 a.Additionally, regions that contribute to R′_(LRS2) and R′_(HRS2) caninclude a bulk material of the storage node 56 a. Additionally, regionsthat contribute to R′_(LRS3) and R′_(HRS3) can include a secondinterface between the storage node 56 a and the bottom metallic contactregion 58 a, a bulk material of the bottom metallic contact region 58 a,and a bulk material of the bottom thermally insulating region 60 a. Itis to be understood that while other regions may also contributesubstantially to the overall resistance between the first and secondterminals 14 a and 14 b, only first through third resistors 62, 64, and66 are represented in the access circuit path 14 for clarity. Inaddition, other circuit paths can exist, for example, through the secondmemory cell 50 b, which are not shown nor discussed for clarity.

In the illustrated embodiment of FIGS. 3A and 3B, by choosing a suitablematerial for the top metallic contact region 54 a as discussed above,the contribution of the first interface between the top electrode andthe storage node 56 a to R′_(LRS1) and R′_(HRS1) can be substantiallyreduced compared to embodiments that do not include the top metalliccontact region 54 a. Similarly, by choosing a suitable material for thebottom metallic contact region 58 a as discussed above, the contributionof the second interface between the bottom electrode and the storagenode 56 a to R′_(LRS3) and R′_(HRS3) can be substantially reducedcompared to embodiments that do not include the bottom metallic contactregion 58 a. This is because interfaces formed by top and bottomthermally insulating regions 52 a and 60 a in contact with thechalcogenide material element 56 a can have substantially higher contactresistances compared interfaces formed by top and bottom metalliccontact regions 54 a and 58 a in contact with the chalcogenide materialelement 56 a.

In one embodiment, the contribution of interfacial resistances of firstand second interfaces to first and third LRS resistances R′_(LRS1) andR′_(LRS3) and first and third HRS resistances R′_(HRS1) and R′_(HRS3) isbelow about 1×10⁻⁶ Ohm cm². In another embodiment, the contribution ofinterfacial resistances of first and second interfaces to first andthird LRS resistances R′_(LRS1) and R′_(LRS3) and first and third HRSresistances R′_(HRS1) and R′_(HRS3) is below about 1×10⁻⁷ Ohm cm². Inyet another embodiment, the contribution of interfacial resistances offirst and second interfaces to first and third LRS resistances R′_(LRS1)and R′_(LRS3) and first and third HRS resistances R′_(HRS1) andR′_(HRS3) is below about 5×10⁻⁸ Ohm cm².

By reducing contributions of first and second interfaces to LRS and HRSresistances as described above in the embodiment of FIGS. 3A and 3B, ahigher ON/OFF ratio can be achieved. This is because as discussed above,the ON/OFF ratio can be proportional to the ratio(R′_(HRS1)+R′_(HRS2)+R′_(HRS3))/(R′_(LRS1)+R′_(LRS2)+R′_(LRS3)), andwhen R′_(HRS2)>>R′_(HRS1)+R′_(HRS3) and R′_(LRS2)>>R′_(LRS1)+R′_(LRS3),the ON/OFF ratio can be dominated by the ratio R′_(HRS2)/R′_(LRS2).Under this circumstance, the ON/OFF ratio can be desirably dominated bythe resistance values of the bulk resistance of the storage node 56 a inSET and RESET states. Thus, from an ON/OFF ratio point of view, animplementation exemplified in FIGS. 3A and 3B can desirably haverelatively low LRS and HRS resistances R′_(LRS1), R′_(LRS3), R′_(HRS1),and R′_(HRS3) of the first and third resistors 62 and 66 and relativelyhigh LRS and HRS resistances R′_(LRS2) and R′_(HRS2) of the secondresistor 64.

Additionally, in the illustrated embodiment of FIGS. 3A and 3B, bychoosing a suitable material for the top metallic contact region 54 a asdiscussed above, higher energy efficiencies of SET and RESET operationscan also be achieved. This is because it may be desirable to haveself-heating of the storage node dominate over heats generated in otherregions. From this standpoint, it may be desirable to haveR′_(HRS2)>>R′_(HRS1)+R′_(HRS3) and R′_(LRS2)>>R′_(LRS1)+R′_(LRS3). Whena voltage V_(ACCESS) is applied between the first and second terminals14 a and 14 b of the access circuit path 14, voltages proportional torespective LRS and HRS resistances can drop across first, second, andthird resistors 62, 64, and 66. In particular, when V_(ACCESS)=V_(RESET)is applied between the first and second terminals 14 a and 14 b, first,second, and third voltages V′_(RESET1), V′_(RESET2), and V′_(RESET3) candrop across first, second, and third resistors 62, 64, and 66,respectively. In addition, the resulting currentI_(RESET)=V_(RESET)/(R′_(LRS1)+R′_(LRS2)+R′_(LRS3)) can result ingeneration of first, second, and third RESET heats Q′_(RESET1)=I_(RESET)²R′_(LRS1) and Q′_(RST2)=I_(RESET) ²R′_(LRS2), and Q′_(RST3)=I_(RESET)²R′_(LRS3), respectively. Thus, by havingR′_(LRS2)>>R′_(LRS1)+R′_(LRS3), more of the access energy is spent onself-heating the chalcogenide material element 56 a compared to theinterfaces.

Similarly, when V_(ACCESS)=V_(SET) is applied between the first andsecond terminals 14 a and 14 b, first, second, and third voltagesV′_(SET1), V′_(SET2), and V′_(SET3) can drop across first, second, andthird resistors 62, 64, and 66, respectively. In addition, the resultingcurrent I_(SET)=V_(SET)/(R′_(HRS1)+R′_(HRS2)+R′_(HRS3)) can result inthe generation of first, second, and third SET heats Q′_(SET1)=I_(SET)²R′_(HRS1) and Q′_(SET2)=I_(SET) ²R′_(HRS2), and Q_(SET3)=I_(SET)²R′_(HRS3), respectively. Thus, by havingR′_(HRS2)>>R′_(HRS1)+R′_(HRS3), more of the access energy is spent onheating the chalcogenide material element 56 a versus the interfaces.

Furthermore, in the illustrated embodiment of FIGS. 3A and 3B, bychoosing a suitable material for top and bottom insulating regions 52 aand 60 a and for first and second sidewall thermal insulators 72 and 74as discussed above still higher energy efficiency of SET and RESEToperations can be achieved by more efficiently confining the heatgenerated in the storage node in response to dissipation of power duringSET and RESET operations. As illustrated in FIGS. 3A and 3B, there canbe heat loss in six directions. A first heat Q′₁ can be lost from thestorage node 56 a in the z-direction towards the column line 20. Asecond heat Q′₂ can be lost from the storage node 56 a in thez-direction towards the row line 22. A third heat Q′₃ can be lost fromthe storage node 56 a in opposite y-directions towards adjacent storagenodes 56 b and 56 h (not shown). A fourth heat Q′₄ can be lost from thestorage node 56 a in opposite x-directions towards adjacent storagenodes 54 c and 54 e (not shown).

First through fourth heats Q′₁-Q′₄ can be characterized as beinginversely proportional to first through fourth thermal resistances Inthe fully-confined array architecture of the cross-point memory array80, contributions to first through fourth thermal resistancesR′_(TH1)-R′_(TH4) can originate from various regions connected to thememory cell 50 a, which can be a target memory cell. For example,regions that contribute to R′_(TH1) and R′_(TH2) can include bulkmaterials of the top and bottom thermally insulating regions 52 a and 60a. Thus, by choosing a suitable material for top and bottom insulatingregions 52 a and 60 a, e.g., a carbon-based material, Q′₁ and Q′₂ can besubstantially reduced compared to embodiments that do not include topand bottom insulating regions 52 a and 60 a.

In this connection, choosing materials having suitable thermalresistivities can be important for minimizing Q′₁ and Q′₂. In oneembodiment, a first ratio of the thermal resistivity corresponding totop thermally insulating regions 52 a-52 c and the thermal resistivitycorresponding to top metallic contact regions 54 a-54 c has a rangebetween about 1 and 500. In another embodiment, the first ratio has arange between about 1 and 200. In yet another embodiment, the firstratio has a range between about 10 and 50. Similarly, a second ratio ofthe thermal resistivity corresponding to bottom thermally insulatingregions 60 a-60 c and the thermal resistivity corresponding to bottommetallic contact regions 58 a-58 c can have similar ranges and values asthe first ratio.

Additionally, regions that contribute to R′_(TH3) can include interfacesand bulk materials formed between the storage node 56 a and sidewalllayers 72-1 to 72-n of first sidewall thermal insulators 72. Similarly,regions that contribute to R′_(TH4) can include interfaces and bulkmaterials formed between the storage node 56 a and sidewall layers74-(n+1) to 72 m of second sidewall thermal insulators 74. Thus, bychoosing suitable materials and numbers of sidewall layers for first andsecond sidewall thermal insulators 72 and 74 as discussed above, Q′₃ andQ′₄ can be substantially reduced compared to embodiments that do notinclude first and second sidewall thermal insulators 72 and 74.

A person skilled in the art will understand that having first and secondsidewall thermal insulators 72 and 74 including a plurality sidewalllayers can increase R′_(TH3) and R′_(TH4) by more than a mere linear sumproportional to thicknesses and thermal resistance values of theindividual layers. This is because the presence of interfaces canincrease the thermal resistance independently of the bulk material. Forexample, having first and second sidewall layers of first and seconddielectric materials having a combined first thickness can have lowerthermal resistance compared to first through tenth sidewall layers ofalternating first and second dielectric materials having a combinedsecond thickness equal to the first thickness.

Similarly, higher R′_(TH3) and R′_(TH4) resulting from having first andsecond sidewall thermal insulators 72 and 74 can reduce program disturbof adjacent cells. As discussed above in connection with FIGS. 2A and2B, program disturb of adjacent cells depend on the increasedtemperature of the adjacent victim cell induced by programming a targetcell. In this connection, minimizing loss of third and fourth heats Q′₃and Q′₄ in FIGS. 3A and 3B, which can be achieved by maximizing R′_(TH3)and R′_(TH4) discussed above, can lead to an improvement in programdisturb performance of the cross-point memory array 80.

According to the embodiments of FIGS. 3A and 3B, column lines 20 have afirst thickness h₁ and the storage node 56 a-c have a third thickness h₃similar to the first and third thicknesses of FIGS. 2A and 2B. Inaddition, top thermally insulating regions 52 a-c and top metalliccontact regions 54 a-c have thicknesses h_(2A) and h_(2B), respectively.In addition, bottom metallic contact regions 58 a-c and bottom thermallyinsulating regions 60 a-c have thicknesses h_(4A) and h_(4B),respectively. In one embodiment, each of the thicknesses h_(2A) andh_(4B) of top and bottom thermally insulating regions 52 a-c and 60 a-ccan be selected to be in the range between 10 and 50 nm. In anotherembodiment, each of the thicknesses h_(2A) and h_(4B) of top and bottomthermally insulating regions 52 a-c and 60 a-c can be selected to be inthe range between 20 and 30 nm. In one embodiment, each of thethicknesses h_(2B) and h_(4A) of top and bottom metallic contact regions54 a-c and 58 a-c can be selected to be in the range between 5 and 30nm. In another embodiment, each of the thicknesses h_(2B) and h_(4A) oftop and bottom metallic contact regions 54 a-c and 58 a-c can beselected to be in the range between 10 and 20 nm. Other dimensions notillustrated, for example the fifth thickness h₅ and the sixth thicknessh₆ can be similar to the dimensions discussed above in connection withFIGS. 2A and 2B.

FIGS. 4A and 4B illustrate cross-sectional views of a portion of across-point memory array 110 viewed from a direction parallel to thex-direction and the y-direction, respectively, according to anotherembodiment. Similar to the cross-point array 80 of FIGS. 3A and 3B, thecross-point array 110 includes memory cells disposed between columnlines 20 and row lines 22. In contrast to the fully-confined arrayarchitecture described above in connection with FIGS. 3A and 3B, thearray architecture in FIGS. 4A and 4B has a top thermally insulatingregion, a top metallic contact region, and a chalcogenide materialelement forming a continuous line along with the column line 20. Inaddition, the array architecture in FIGS. 4A and 4B has bottomelectrodes forming a thin wall structure extending in the row direction.The array architecture is referred to herein as a wall arrayarchitecture.

According to the illustrated embodiment in FIGS. 4A and 4B, a memorycell includes a bottom electrode, a chalcogenide material elementdisposed on the bottom electrode, and a top electrode disposed on thechalcogenide material element. The top electrode includes a topthermally insulating region over the first electrode, where thethermally insulating region comprises carbon and has a first thermalresistivity. Top electrode additionally includes a top metallic contactregion interposed between the chalcogenide material element and thethermally insulating region, where the metallic contact layer has asecond thermal resistivity lower than the first thermal resistivity.

Additionally, the chalcogenide material element has top and bottomsurfaces and first and second sidewalls extending between top and bottomsurfaces, and the memory cell further includes a plurality of sidewallthermal insulators formed over first and second sidewalls.

The cross-point memory array 110 of FIGS. 4A and 4B includes memorycells 90 a-90 c. Memory cells 90 a-90 c include bottom electrodes 98 aand 98 b disposed on row lines (not shown). Bottom electrodes 98 a and98 c in FIG. 4B form a thin wall extending in the x-direction having awall length similar to the fourth lateral dimension in the x-directiond_(4b) corresponding to the bottom electrodes 36 a and 36 c in FIG. 2B.However, unlike FIG. 2B, the bottom electrodes 98 a and 98 b in FIG. 4Ahave a wall thickness substantially thinner than the fourth lateraldimension in the y-direction d_(4a) corresponding to the bottomelectrodes 36 a and 36 b in FIG. 2A. The wall thickness can be anysuitable thickness substantially less than the fourth lateral dimensiond_(4a) and chosen to dissipate sufficient energy to as a heater forchanging the phase of the chalcogenide material element during eitherone or both of SET and RESET operations. For example, in one embodiment,the wall thickness is between about 10% and about 50% of the fourthlateral dimension d₄ a. In another embodiment, the wall thickness isbetween about 20% and about 40% of the fourth lateral dimension d₄ a,for instance about 25%. Due to the thin wall configuration, the bottomelectrodes 98 a-98 c dissipates a substantial amount of energy in bothSET and RESET operations and can function as a heater for thechalcogenide material element.

Additionally, in contrast to the fully-confined array architecturedescribed above in connection with FIGS. 3A and 3B, the wall arrayarchitecture in FIGS. 4A and 4B has chalcogenide material elements 96 aand 96 c disposed on bottom electrodes 98 a and 98 c, top metalliccontact regions 94 a and 94 c disposed on chalcogenide material elements96 a and 96 c, and top thermally insulating regions 92 a and 92 cdisposed on metallic contact regions 94 a and 94 c. In this embodiment,chalcogenide material elements 96 a and 96 c, top metallic contactregions 94 a and 94 c, and top thermally insulating regions 92 a and 92c form continuous lines along with the column line 20. In oneembodiment, chalcogenide material elements 96 a-96 c can be storagenodes In this embodiment, there can be selector nodes coupled elsewhere(not shown in FIGS. 4A and 4B), for example, coupled to bottomelectrodes 98 a-98 c. The selector nodes can be part of various selectordevices described above, including, for example, BJTs and OTSs.

Top metallic contact regions 94 a-94 c include suitable conductive andsemiconductive materials similar to top metallic contact regions ofFIGS. 3A and 3B. In addition, top thermally insulating regions 92 a-92 ccan include carbon in similar forms as in top thermally insulatingregions of FIGS. 3A and 3B, and can be formed using similar processingtechniques.

According to the illustrated embodiment of FIG. 4B, chalcogenidematerial elements 96 a and 96 c have top and bottom surfaces and firstand second sidewalls extending between top and bottom surfaces along thez-direction. In the wall array architecture of the cross-point memoryarray 110, first and second sidewalls extend in the y-direction to spanone column line dimension in the y direction (which is similar to thefirst lateral dimension in the y-direction d_(1a) in FIG. 2A, notillustrated here for clarity). In this configuration, memory cells 90 aand 90 b include a plurality of first sidewall thermal insulators 104formed over first and second sidewalls. Similar to FIG. 3B, theplurality of first sidewall thermal insulators 104 includes a suitablenumber of layers to provide sufficient thermal insulation to improveprogramming efficiency of the chalcogenide material element of a targetmemory cell, and to provide improved immunity against program disturb,as discussed above in connection with FIGS. 2A and 2B. In the exampleembodiment of FIG. 4B, the chalcogenide material element of the targetcell can be 96 a, and the chalcogenide material element of the victimcell can be 96 b along a common row line 22 (not shown). Each of thefirst sidewall thermal insulators 104 can include first through nthsidewall layers, similar to as discussed in FIGS. 3A and 3B. Inaddition, the materials, thicknesses, and the number of first throughnth sidewall layers of the first sidewall thermal insulators 104 can besimilar to those described for the first sidewall thermal insulators inFIGS. 3A and 3B. In addition, the sidewall layers of the second sidewallthermal insulators 104 can be formed using similar techniques as thosedescribed for the first sidewall thermal insulators in FIGS. 3A and 3B.

Although this invention has been described in terms of certainembodiments, other embodiments that are apparent to those of ordinaryskill in the art, including embodiments that do not provide all of thefeatures and advantages set forth herein, are also within the scope ofthis invention. Moreover, the various embodiments described above can becombined to provide further embodiments. In addition, certain featuresshown in the context of one embodiment can be incorporated into otherembodiments as well. Accordingly, the scope of the present invention isdefined only by reference to the appended claims.

What is claimed is:
 1. A method of fabricating an electronic devicecomprising: forming a first electrode; forming a phase change materialelement on the first electrode; and forming a second electrode on thephase change material element; and forming a metallic access line on thesecond electrode, wherein forming the second electrode comprises:forming a metallic contact region on the phase change material element,and forming a thermally insulating region comprising carbon on themetallic contact region, the thermally insulating region having a firstthermal resistivity that is greater than a second thermal resistivity ofthe metallic contact region, wherein forming the first electrodecomprises: forming a second thermally insulating region comprisingcarbon; and forming a second metallic contact region interposed betweenthe phase change material element and the second thermally insulatingregion, the second thermally insulating region having a third thermalresistivity that is greater than a fourth thermal resistivity of thesecond metallic contact region.
 2. The method of claim 1, whereinforming the second electrode and forming the phase change materialelement comprise providing dimensions and electrical resistivities suchthat an electrical resistance of a bulk material of the phase changematerial element is greater than a combined electrical resistance of abulk material of the thermally insulating region, a bulk material of themetallic contact region and an interface between the metallic contactregion and the phase change material element.
 3. A method of fabricatingan electronic device comprising: forming a first electrode; forming aphase change material element on the first electrode; forming a secondelectrode on the phase change material element, the second electrodecomprising a metallic contact region on the phase change materialelement and a thermally insulating region comprising carbon on themetallic contact region, wherein the thermally insulating region has afirst thermal resistivity and the metallic contact region has a secondthermal resistivity lower than the first thermal resistivity; andforming an access line on the second electrode and extending in a firstdirection, wherein the access line and at least the second electrodehave a same nominal width measured in a second direction crossing thefirst direction.
 4. The method of claim 3, wherein the phase changematerial element and the first electrode have the same nominal width. 5.The method of claim 3, further comprising forming a second access linedisposed under the phase change material element and extending in thesecond direction.
 6. The method of claim 1, wherein the second thermalresistivity of the metallic contact region is lower than the firstthermal resistivity by a ratio of at least
 10. 7. The method of claim 6,wherein the metallic contact region comprises tungsten.
 8. The method ofclaim 7, wherein the metallic contact region consists essentially oftungsten.
 9. The method of claim 1, wherein the thermally insulatingregion consists essentially of carbon.
 10. The method of claim 1,wherein forming the second electrode includes contacting the metalliccontact region to the phase change material element.
 11. The method ofclaim 1, further including forming sidewall thermal insulators oversidewalls of the phase change material element.
 12. The method of claim11, wherein forming the sidewall thermal insulators includes formingfirst sidewall layers on the sidewalls of the phase change materialelement, and further forming second sidewall layers on first sidewalllayers.
 13. The method of claim 1, wherein the second thermallyinsulating region is formed of the same material as the thermallyinsulating region.
 14. The method of claim 1, wherein the secondmetallic contact region is formed of the same material as the metalliccontact region.
 15. The method of claim 1, wherein forming the firstelectrode comprises forming a wall structure having a wall width along afirst horizontal direction and a wall thickness along a secondhorizontal direction substantially perpendicular to the first horizontaldirection, wherein the wall thickness is substantially smaller than thewall width.
 16. A method of fabricating an electronic device comprising:forming a first access line extending in a first direction, a secondaccess line extending in a second direction crossing the firstdirection, and a phase change material element vertically interposedbetween the first access line and the second access line; and forming anelectrode vertically interposed between the phase change materialelement and the first access line, wherein the electrode comprises athermally insulating region comprising carbon and having a first thermalresistivity and a metallic contact region interposed between the phasechange material element and the thermally insulating region, themetallic contact region having a second thermal resistivity lower thanthe first thermal resistivity, wherein forming the electrode includespatterning and etching to define at least one dimension of the electrodeand the first access line within a single photo mask level, such thatthe electrode and the first access line have a same first nominallateral dimension in the second direction.
 17. The method of claim 16,wherein forming the electrode further includes, within another secondphoto mask level, further patterning and further etching to defineanother dimension of the electrode along with the second access linewithin another single photo mask level, such that the electrode and thesecond access line have a same second nominal lateral dimension in thefirst direction.
 18. The method of claim 16, further including forming asecond electrode interposed between the phase change material elementand the second access line.
 19. The method of claim 18, wherein thesecond electrode comprises a second thermally insulating regioncomprising carbon and having a third thermal resistivity and a secondmetallic contact region interposed between the phase change materialelement and the second thermally insulating region, the second metalliccontact region having a fourth thermal resistivity lower than the thirdthermal resistivity.
 20. The method of claim 18, wherein the phasechange material element is a phase change memory element.
 21. The methodof claim 20, further comprising forming a selector element interposedbetween the first access line and the second access line, wherein thesecond electrode is interposed between the phase change memory elementand the selector element.
 22. The method of claim 21, wherein theselector element is formed of a chalcogenide material different from thephase change memory element.
 23. The method of claim 16, wherein each ofthe first access line and the second access line comprises metal lines.